The study of inorganic nanometer-scale materials with hollow closed-cage structures is a rapidly growing field. The first inorganic fullerene-like nanoparticles (IF) and nanotubes (INT) of WS2 and MoS2 [Tenn et al., Nature 1992, 360, 444-446; Margulis et al., Nature 1993, 365, 113-114] were obtained by heating thin metal films (W or Mo) in the presence of gaseous H2S. A variety of layered transition metal chalcogenides such as TiS2, NbS2, TaS2 and other inorganic layered materials have demonstrated the ability to form IF/INT structures, and a number of synthetic routes have been developed, e.g., chemical vapor transport using bromine for INT-MoS2, a bismuth-catalyzed vapor-liquid-solid method for SnS2 nanotubes and superstructure SnS—SnS2 nanotubes.
The most promising method for high yields of almost defect-free IF-MoS2, IF—WS2 IF and INT-WS2 is sulfidization of the respective metal oxides under reducing conditions. For IF-WS2, WO3 spherical nanoparticles were used as solid precursors in a process where the reaction temperature was lower than the sublimation temperature of WO3, so that the kinetically controlled reaction proceeded according to a solid-gas mechanism. INT-WS2 were also prepared by a two-step sulfidization of WO3-x nanoparticles at 800-900° C.: the rapid growth of long non-volatile W18O49 nanowhiskers followed by sulfidization under reducing conditions in the same reactor.
The main obstacle for obtaining INT-MoS2 from the respective oxide is the lack of an anisotropic MoO3-x phase that could potentially promote the growth of non-volatile MoO3-x nanowhiskers in a high-temperature reaction. MoO2 nanowhiskers and nanotubes were obtained by flame heating of a molybdenum tip at 2500° C. in an acetylene-oxygen rich atmosphere [Merchan-Merchan et al., Chem. Phys. Lett. 2006, 422, 72-77] where the high temperature, strong temperature gradient and chemical environment promoted the growth of 1-D nanostructures.
Previously, Yamazoe et al. reported that vanadium substitution stabilized a Mo17O47 phase [Yamazoe et al., Acta Chem. Scand., Ser. A 1975, 29, 404-8]. Similarly, the Mo5O11 phase was stabilized by adding minute amounts of titanium, niobium and tantalum [Portemer et al., Solid State Chem 1993, 103, 403-414; Kihlborg, L. Acta Chem. Scand. 1969, 23, 1834-1835]. Hence, it was hypothesized that different metals which stabilize asymmetric MoO3-x phases could lead to the growth of nanowhiskers and, subsequently, INT-MoS2.
Metal catalysts are widely used for promoting the growth of 1-D nanoparticles. The most common method is vapor-liquid-solid growth, yielding various nanowires, carbon nanotubes and INT with control of their composition and dimensions. The synthesis of carbon nanotubes has also been realized using homogeneous metal catalysts.
The most common methods for sulfidization of metal oxide powder involve using H2S gas, which is flammable and toxic, in a reducing atmosphere. Furthermore, the generation of metal selenide and metal telluride nanoparticles requires even more toxic and unstable precursors. These observations motivate the search for other sources of sulfide (selenide and telluride), as well as alternative synthetic techniques.
Solar ablation as a technique for IF/INT synthesis offers the advantage of permitting a large reaction volume in combination with a high vapor pressure of the reactants at reactor temperatures up to ˜3000K with an ultra-hot annealing environment. Moreover sharp gradients in heat flux and temperature are created, estimated as high as 104 K/cm. This method has been employed successfully for the synthesis of carbon fullerenes, carbon nanotubes, and an assortment of inorganic nanoparticles [Wiesel et al., Nano Research 2009, 2, 416-424; Levy et al., Isr. J. Chem. 2010, 50, 417-425; Flamant et al., Sol. Energy 1999, 66, 117-132; Albu Yaron et al., Adv. Mater 2006, 18, 2993-2996; Albu-Yaron et al., Angew. Chem. Int. Ed. 2011, 50, 1810-1814].